Broad bandwidth optical transmission systems have received a great deal of attention in recent years. Such systems require broad bandwidth optical amplifiers to achieve transmission of high capacity wavelength division multiplexed signals. A type of optical amplifier that is sometimes employed is a so-called distributed amplifier in which signal amplification occurs along the signal transmission path. An example of a distributed amplifier is a Raman amplifier.
Raman amplification is accomplished by introducing the signal and pump energies along the same optical fiber. The pump and signal may be copropagating or counterpropagating with respect to one another. A Raman amplifier uses stimulated Raman scattering, which occurs in silica fibers when an intense pump beam propagates through it. Stimulated Raman scattering is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (i.e., optical phonons). That is, pump energy of a given wavelength amplifies a signal at a longer wavelength. The relationship between the pump energy and the Raman gain for a silica fiber is shown in FIG. 1. The particular wavelength of the pump energy that is used in this example is denoted by reference numeral 1. As shown, the gain spectrum 2 for this particular pump wavelength is shifted in wavelength with respect to the pump wavelength. As FIG. 1 indicates, the bandwidth of the Raman amplifier is limited. For example, the bandwidth of the amplifier shown in FIG. 1 is only about 20 nm at a gain of 10 dB.
U.S. Appl. Ser. No. 09/030,994, filed Feb. 26, 1998 and entitled WIDE BANDWIDTH RAMAN AMPLIFIER EMPLOYING A PUMP UNIT GENERATING A PLURALITY OF WAVELENGTHS, discloses a Raman amplifier that has an increased bandwidth. This result is accomplished by providing two pump sources providing pump energy at two or more different wavelengths. As shown in FIG. 2, pump energy supplied at a wavelength denoted by reference numeral 40 generates gain curve 42 while pump energy supplied at a wavelength denoted by reference numeral 41 generates gain curve 43. The composite gain spectrum, indicated by curve 44, has a bandwidth that is greater than either of the individual gain curves 42 and 43.
Unfortunately, the bandwidth of the Raman amplifier disclosed in the previously mentioned reference cannot be increased beyond a limited amount. This limitation arises because it is not possible to provide pump energy that spectrally overlaps the signal. As a result, the wavelength separation between the pump wavelengths 40 and 41 is limited to about 100 nm, since at greater separations the pump wavelength 41 will overlap the gain curve 42 of pump wavelength 40.
Accordingly it would be desirable to provide a Raman amplifier in which the bandwidth could be substantially increased over the bandwidth that can be achieved by the Raman amplifier disclosed in U.S. Appl. Ser. No. 09/030,994.